Editing Polychlorinated biphenyl (section)

==Environmental transport and transformations==
PCBs have entered the environment through both use and disposal. The environmental fate of PCBs is complex and global in scale.<ref name="unep"/>

===Water===
Because of their low [[vapour pressure]], PCBs accumulate primarily in the [[hydrosphere]], despite their [[hydrophobicity]], in the organic fraction of [[soil]], and in [[organism]]s including the human body.<ref>{{cite journal | vauthors = Norström K, Czub G, McLachlan MS, Hu D, Thorne PS, Hornbuckle KC |display-authors=3| title = External exposure and bioaccumulation of PCBs in humans living in a contaminated urban environment | journal = Environment International | volume = 36 | issue = 8 | pages = 855–861 | date = November 2010 | pmid = 19394084 | pmc = 2891214 | doi = 10.1016/j.envint.2009.03.005 | bibcode = 2010EnInt..36..855N }}</ref> The hydrosphere is the main reservoir. The immense volume of water in the oceans is still capable of dissolving a significant quantity of PCBs.<ref>{{cite web |title=A Look at How Long-Banned PCBs Persist in the Ocean |url=https://eos.org/research-spotlights/a-look-at-how-long-banned-pcbs-persist-in-the-ocean |website=Eos|date=7 May 2019 }}</ref>

As the pressure of ocean water increases with depth, PCBs become heavier than water and sink to the deepest ocean trenches where they are concentrated.<ref>{{cite news|url=https://www.economist.com/science-and-technology/2017/02/18/nasty-chemicals-abound-in-what-was-thought-an-untouched-environment|title=Nasty chemicals abound in what was thought an untouched environment|newspaper=The Economist|access-date=10 April 2022|date=18 February 2017}}</ref>

===Air===
A small volume of PCBs has been detected throughout the Earth's atmosphere. The atmosphere serves as the primary route for global transport of PCBs, particularly for those congeners with one to four chlorine atoms.<ref>{{cite book |doi=10.1016/B0-12-369400-0/00785-7 |chapter=Polychlorinated Biphenyls (PCBS) |title=Encyclopedia of Toxicology |date=2005 |last1=Kulkarni |first1=Swarupa G. |last2=Mehendale |first2=Harihara M. |pages=509–511 |isbn=978-0-12-369400-3 }}</ref>

In the atmosphere, PCBs may be degraded by [[hydroxyl radical]]s, or directly by [[photolysis]] of carbon–chlorine bonds (even if this is a less important process).<ref>{{cite journal |vauthors=Xin ML, Yang JW, Li Y |title=The mechanism for enhanced oxidation degradation of dioxin-like PCBs (PCB-77) in the atmosphere by the solvation effect |journal=Chemistry Central Journal |volume=11 |issue=1 |pages=61 |date=July 2017 |pmid=29086899 |pmc=5503851 |doi=10.1186/s13065-017-0291-3 |doi-access=free }}</ref>

Atmospheric concentrations of PCBs tend to be lowest in rural areas, where they are typically in the [[picogram]] per cubic meter range, higher in suburban and urban areas, and highest in city centres, where they can reach 1&nbsp;[[nanogram|ng]]/m<sup>3</sup> or more.<ref>{{cite journal |last1=Diamond |first1=Miriam L. |last2=Melymuk |first2=Lisa |last3=Csiszar |first3=Susan A. |last4=Robson |first4=Matthew |title=Estimation of PCB Stocks, Emissions, and Urban Fate: Will our Policies Reduce Concentrations and Exposure? |journal=Environmental Science & Technology |date=15 April 2010 |volume=44 |issue=8 |pages=2777–2783 |doi=10.1021/es9012036 |pmid=20170162 |doi-access=free }}</ref> In [[Milwaukee]], an atmospheric concentration of 1.9&nbsp;ng/m<sup>3</sup> has been measured, and this source alone was estimated to account for 120&nbsp;kg/year of PCBs entering [[Lake Michigan]].<ref>{{cite journal |vauthors=Wethington DM, Hornbuckle KC |title=Milwaukee, WI, as a source of atmospheric PCBs to Lake Michigan |journal=Environmental Science & Technology |volume=39 |issue=1 |pages=57–63 |date=January 2005 |pmid=15667075 |doi=10.1021/es048902d |bibcode=2005EnST...39...57W}}</ref> In 2008, concentrations as high as 35&nbsp;[[nanogram|ng]]/m<sup>3</sup>, 10&nbsp;times higher than the EPA guideline limit of 3.4&nbsp;ng/m<sup>3</sup>, have been documented inside some houses in the U.S.<ref name="rudel2008"/>

Volatilization of PCBs in soil was thought to be the primary source of PCBs in the atmosphere, but research suggests ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere.<ref>{{cite journal |vauthors=Jamshidi A, Hunter S, Hazrati S, Harrad S |title=Concentrations and chiral signatures of polychlorinated biphenyls in outdoor and indoor air and soil in a major U.K. conurbation |journal=Environmental Science & Technology |volume=41 |issue=7 |pages=2153–2158 |date=April 2007 |pmid=17438756 |doi=10.1021/es062218c |bibcode=2007EnST...41.2153J}}</ref>

===Biosphere===
In the [[biosphere]], PCBs can be degraded by the [[sun]], [[bacteria]] or [[eukaryote]]s, but the speed of the reaction depends on both the number and the disposition of chlorine atoms in the molecule: less substituted, ''meta''- or ''para''-substituted PCBs undergo biodegradation faster than more substituted congeners.{{Citation needed|date=May 2013}}

In bacteria, PCBs may be dechlorinated through [[reductive dechlorination]], or oxidized by [[dioxygenase]] enzyme.{{Citation needed|date=May 2013}} In eukaryotes, PCBs may be oxidized by the [[cytochrome P450]] enzyme.<ref>{{cite journal | vauthors = Tiedje JM, Quensen JF, Chee-Sanford J, Schimel JP, Boyd SA |display-authors=3| title = Microbial reductive dechlorination of PCBs | journal = Biodegradation | volume = 4 | issue = 4 | pages = 231–240 | date = 1993–1994 | pmid = 7764920 | doi = 10.1007/BF00695971 | s2cid = 2596703 }}</ref>
[[File:Biomagnification.svg|thumb|[[Biomagnification]] is the increasing concentration of a substance, such as a toxic chemical, in the tissues of tolerant organisms at successively higher levels in a food chain.]]
Like many lipophilic toxins, PCBs undergo [[biomagnification]] and [[bioaccumulation]] primarily due to the fact that they are easily retained within organisms.<ref name=Schmidt>{{cite journal | vauthors = Schmidt C, Krauth T, Wagner S | title = Export of Plastic Debris by Rivers into the Sea | journal = Environmental Science & Technology | volume = 51 | issue = 21 | pages = 12246–12253 | date = November 2017 | pmid = 29019247 | doi = 10.1021/acs.est.7b02368 | bibcode = 2017EnST...5112246S | url = http://oceanrep.geomar.de/43169/4/es7b02368_si_001.pdf }}</ref><ref>{{cite journal | vauthors = Ferreira I, Venâncio C, Lopes I, Oliveira M | title = Nanoplastics and marine organisms: What has been studied? | journal = Environmental Toxicology and Pharmacology | volume = 67 | pages = 1–7 | date = April 2019 | pmid = 30685594 | doi = 10.1016/j.etap.2019.01.006 | bibcode = 2019EnvTP..67....1F | hdl = 10773/37415 | s2cid = 59306677 | hdl-access = free }}</ref>

Plastic pollution, specifically [[microplastics]], are a major contributor of PCBs into the biosphere and especially into marine environments. PCBs concentrate in marine environments because freshwater systems, like rivers, act as a bridge for plastic pollution to be transported from terrestrial environments into marine environments.<ref>{{Cite journal| vauthors = Ma P, Wei Wang M, Liu H, Feng Chen Y, Xia J |display-authors=3|date=2019-01-01|title=Research on ecotoxicology of microplastics on freshwater aquatic organisms|journal=Environmental Pollutants and Bioavailability|volume=31|issue=1|pages=131–137|doi=10.1080/26395940.2019.1580151 |doi-access=free|bibcode=2019EnvPB..31..131M }}</ref> It has been estimated that 88–95% of marine plastic is exported into the ocean by just 10 major rivers.<ref name=Schmidt/>

An organism can accumulate PCBs by consuming other organisms that have previously ingested PCBs from terrestrial, freshwater, or marine environments. The concentration of PCBs within an organism will increase over their lifetime; this process is called bioaccumulation. PCB concentrations within an organism also change depending upon which [[trophic level]] they occupy. When an organism occupies a high trophic level, like [[orcas]] or [[human]]s, they will accumulate more PCBs than an organism that occupies a low trophic level, like [[phytoplankton]]. If enough organisms with a trophic level are killed due to the accumulation of toxins, like PCB, a [[trophic cascade]] can occur.

PCBs can cause harm to human health or even death when eaten.<ref>{{cite news | vauthors = Faber H |title=Hunters who eat ducks warned on PCB hazard |url=https://www.nytimes.com/1981/10/08/nyregion/hunters-who-eat-ducks-warned-on-pcb-hazard.html |newspaper=New York Times |date=October 8, 1981}}</ref> PCBs can be transported by birds from aquatic sources onto land via feces and carcasses.<ref>{{cite journal | vauthors = Rapp Learn J |title=Seabirds Are Dumping Pollution-Laden Poop Back on Land |website=Smithsonian.com |date=November 30, 2015 |url=http://www.smithsonianmag.com/science-nature/seabirds-are-dumping-pollution-laden-poop-back-land-180957288/?no-ist|access-date=30 November 2015}}</ref>